German Offenlegungsschrift No. 1,933,128 shows an arrangement for diffusing doping materials into a semiconductor material wherein a tube of crystalline gas-tight semiconductor material is used as a diffusion container which can be heated by way of applying thereto a voltage directly or with the help of high-frequency energy. The tube serving as heating member may have its opposed ends connected with electrodes or may be circumferentially surrounded by an induction heating coil. To facilitate a heating of the tube by induction heating, a ring made of conductive material, such as graphite, is applied about the tube. When the tube is directly heated by an applied voltage, the voltage becomes dependent on the conductivity of the tube semiconductor material, independently of the tube dimensions in order to achieve diffusion temperatures. In order to use relatively low voltages for the initiation of the heating process, the above-mentioned reference suggests the use of highly doped semiconductor material for the diffusion tube, which can be produced relatively cheaply. When a predeterminable heating temperature is reached, the conductivity of the tube becomes independent of the doping of the semiconductor material and dependent upon the tube dimensions.
Due to the gas-phase deposition technique employed for the production of a diffusion tube as taught, for instance, in German Pat. No. 1,805,870, wherein semiconductor material from a gaseous semiconductor compound is precipitated onto the outer surface of a carrier member such as graphite and the carrier member is removed without destroying the semiconductor material layer, the production of pure, gas-tight tubes made of semiconductor material, in particular silicon, have become possible. Such pure tubes are heatable by direct current passage only with preheating.
When doped semiconductor material is used for the diffusion tube as mentioned in German Offenlegungsschrift No. 1,933,128, it is possible to eliminate such a pre-heating sequence and to heat such tube directly. However, this arrangement suffers from the fact that an undesired interaction between the high doping and a semiconductor component member can occur during a diffusion operation utilizing such tube.
A directly heatable silicon tube is provided in German Offenlegungsschrift No. 2,253,411, which tube is produced in such a way that at least two layers are successively deposited on the circumferential surface portions of a carrier member provided for such deposition, and the outermost deposited layer is provided with a doping, while the innermost layer consists of highly pure silicon. In a tube of such construction, the tube outer doping does not influence a semiconductor member receiving a diffusion treatment therewithin. However, the problem of providing an electric contact for such a tube when such is used as a tubular oven for semiconductor diffusion processes is not solved in this Offenlegungsschrift.
An electrical contact arrangement for a silicon tube which is to be heated directly is described in German Offenlegungsschrift No. 2,340,225. Here, the silicon tube which is being heated is provided with a doping in the areas where current contacts are to be applied in the assembled oven. A lacquer which contains phosphorus is applied to tube areas to be doped. Upon subsequent heating, the lacquer volatilizes without residue, and the doping material is diffused into the silicon tube. Heating collars made of aluminum or an aluminum alloy are used as current contacts, and they are placed onto a support of graphite felt. In this construction, the problem of achieving a satisfactory electric contacting of a directly heatable silicon tube has not been solved with respect to temperature constancy and heating duration. This construction interferes essentially with the reproducability of diffusion processes conducted in the assembled oven.